Water-based gravure inks, which are now used extensively in carton, gift wrap and decorative product printing, have significantly different rheology, solids content, drying characteristics and other doctor blade wiping-related properties than the solvent-based inks they are replacing. Water-based inks typically have higher surface tension, decreased wetting properties and much higher pigment, binder and film forming solid content, which combine with the resultant lower liquid content to cause higher internal friction and static with restricted flow and lubrication properties of the wet ink on the press. Water-based inks employ such higher solids content with shallower etched cylinders to apply adequate pigment using thinner films so that less water is left to be removed during drying. These inks create relatively higher hydraulic force profiles at the doctor blade tip than did solvent based inks and lead to wiping defects at desired run speeds.
Many gravure doctor blade systems now in service using water based inks and other, high solid content or high viscosity inks and coatings, were originally designed for use with solvent based inks. The relatively greater hydraulic forces from these newer water based and other higher solids content inks on the tip of the doctor blade at the desired high press speeds tend to lift the doctor blade causing the passage of a visible film of ink under the blade in non-printing areas. This problem is referred to as haze.
Attempts have been made to compensate in solvent gravure ink systems for these changes in ink characteristics by applying additional force to the doctor blade to prevent haze. This typically results in greater doctor blade tip deflection and a smaller than desired contact angle between the blade tip and the cylinder. Additional force on the doctor blade typically causes more haze after a brief wear-in period. It can further cause dried ink entrapment behind the blade and various resulting print defects. Also, the higher solid content and greater resulting friction eventually causes additional gravure cylinder wear.
One attempted solution to the doctor blade problem has been the development of so-called xe2x80x9cwear proofxe2x80x9d doctor blades that can present small working tips to shear the ink from the gravure cylinder. For example, some doctor blades are now made of a special alloy steels with greater wear resistance than the knife steel and carbon steel blades conventionally used in solvent-based ink gravure cylinder blades. Some have special tip geometries that are supposed to improve shear performance. The success of these new blades is often limited by the performance of the remainder of the blade system, particularly the blade holder. The improved wear performance of such new blades does not overcome fundamental problems with doctor blade wiping force requirements, geometry and lubrication. Nor do they help alleviate the cylinder wear problem.
New gravure presses are often provided with a prewipe doctor blade of conventional construction, which is supported off the gravure roll journals, or with a separate rubber roll actually running in the ink sump to indirectly ink the gravure cylinder roll. Neither approach is easily or inexpensively implemented in older inking systems designed for use with conventional, solvent based inks.
In one aspect the invention is an improved doctor blade system in a rotary printing press print station including a print cylinder having an axial direction and a circumferential outer surface and an ink supplier on a side of the print cylinder, the improved doctor blade system comprising: an elongated blade holder extending parallel to the cylinder and pivotally supported on one side of the cylinder for rotation towards and away from the cylinder; an elongated doctor blade mounted on the holder for line contact with axial length of the outer circumferential surface of the cylinder when the holder is rotated sufficiently towards the cylinder; and a prewipe blade mounted on the holder below the doctor blade for contact with the axial length of the outer cylindrical surface of the cylinder below the line contact of the doctor blade with the cylinder so as to wipe some of the ink from the length of the cylinder surface before the length of the surface reaches the doctor blade.
In another aspect, the invention is an improved doctor blade system in a rotary printing press print station including a print cylinder having an axial direction and a circumferential outer surface and an ink supplier located on a side of the cylinder, the improved doctor blade system comprising: an elongated blade holder extending parallel to the cylinder and supported on one side of the print cylinder; a first clamp on the holder configured to releasably hold a generally flat, elongated doctor blade and located to position the doctor blade in line contact with an axial length of the outer circumferential surface of the print cylinder; and a second clamp on the holder configured to releasably hold a generally flat, elongated prewipe blade and located to position the prewipe blade against the length of the outer circumferential surface of the print cylinder below the line contact of the doctor blade so as to remove some ink from the length of the surface before the length of the surface reaches the doctor blade.